Battery Voltage spec

Maybe a bit of a dumb question, but I thought I'd ask because I'm interested!
You can buy various tools which are operated with rechargeable batteries. And there is quite a range of battery voltages to choose between - 7.2, 12, 18, 20, 24, etc.
Question is, is a higher (or lower) voltage better (or worse)? Higher voltage implies lower current to achieve the same power rating.
Andrew
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On Wed, 20 Aug 2003 21:38:45 +0100, Andrew McKay

Broadly, yes, although there is appropriateness.
For example a 9.6v screwdriver may be fine but not a 9.6v SDS drill :-)
More to the point is the battery technology and quality (they are not all the same) , the electronic control of the motor if used, the charging arrangements for the batteries and the mechanics of the tool.
In other words this can be a marketing scam. It's possible to put together a tool with crappy battery packs of 24v, a cheap charger that overcharges or doesn't charge quickly enough and a junk motor and mechanics.
I've found that this is definitely an area where buying cheap unbranded tools based on battery voltage is an unwise move.
One of my most used tools is a Makita 6228, which is a 14v drill. This has superb electronic speed control and torque with two geared ranges. The charger will recharge the battery packs in an hour, and most deals for these (at around 140 inc.) come with 3 batteries. This means that you can keep going continuously in any practical work that I've ever done.
.andy
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You've three variables really:
1: the instantaneous power requirement of the tool
2: the Voltage of the battery pack
3: the "Ampere-hour" rating of the battery pack.
Put simply, 2*3 is the "power available" in the pack, so it can be seen that increasing either 2 or 3 will result in more power being available to the tool. So, two tools of a similar power useage but different battery ratings will be able to be used for differing lengths of time before requiring a recharge, likewise two tools with different power useage but similar battery ratings.
The only other thing to worry about is that - in theory - the higher the battery voltage, the more "torque" can be available at the motor, so an 18V combi drill *should* find making a hole in a masonry wall easier than a 12V combi drill. In practice there are other factors which affect this ability which include the quality of the cells in the battery pack (particularly their "internal resistance"), the quality of motor, of control electronics and especially of the mechanical design of the tool; the gearbox for example.
The main problem I've found when comparing battery tools is that factor 1: is very rarely quoted in the literature available, and the factors mentioned in the above paragraph are only known when you have used the tool for a while. The only info to go on (in the absence of reccommendations or otherwise from places such as uk.d-i-y) is therefore the battery ratings and, generally, the higher the Voltage, the higher power the tool. Some cheap brands manage to do this by compromising on the quality of the cells, or perhaps by fitting cells with lower Ampere-hour ratings so that you have to recharge them more often.
Bit convoluted, and undoubtedly someone will come up with a more lucid explanation, but I hope this has helped a bit :-)
Hwyl!
Martin.
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They all seem to use the same sized cells - called sub C size. So the only way to increase capacity is to increase the voltage - you can't parallel these types of cells.
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Dave Plowman wrote:

The capacity of the sub c cells can vary by quite a margin however. Cheap and nasty ones as found it the budget tools may only be good for 1.2Ah. The better end of the market can offer twice that capacity not to mention closer cell matching, and lower internal resistance, better cycle life and discharge characteristics.
Not all battery packs are equal - and to an extent you get what you pay for.
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Absolutely. I've re-celled several cheapo packs, and the difference in the tool performance is noticeable.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Got it in one.
Cheap battery tools equals cheap battery and cheap charger. Either the charger fails to charge or boils the battery, sooner or later. More expensive tools tend to work until battery fatigue.
Either way though buying a complete new machine is cheaper than replacing parts.
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CPC are doing a 24v hammer drill with two batteries for less than 30 - cheap enough to discard it when the batteries go
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Dave Plowman wrote:

Actualy you can for discharge, but not for charge:) Well even then...Oh well. Its not practicable for power tools. :)
Otherwise you are totally correct.
Sub 'C's are rated for something like 30-100A peak discharge rate, for Nicads, and maybe 30A only for NiMh.
So in practice power available is very much a function of the number of cells assuming the motor is rated to take the correct current draw.
SUB 'C's come in various capacities too - the originals were something like 1.2A/h but modern NiMh cells are pushing 3A/h.
With a motor efficiency (these are real cheapo motors) in the 50-60% range a 10 cell unit (12V) at 30A is roughly 360W in and about 200W out.
Or a bit more than 1/4bhp. Of course at this current the pack will last only 6 minutes flat out :)
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more volts equal more power if all the other factors are the same. the most used drill I have is a 9,6volt hitachi (brilliant for screwdriving and holes in wood) second most used is a 12volt elu impact screwdriver, 7.2makita then come the also rans 18v cheapie, 24volt cheapie, 12v b&D 12vbosch. So you can see the quality and usability are the most important factors, then when you have a make that works well, get the highest voltage that does what you need with the most practical weight.
mrcheerful
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On Wed, 20 Aug 2003 21:38:45 +0100, Andrew McKay
Judge it by weight of the pack. Given the variation in cell quality, weight is as good a guide as any.
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